Electro-Physical Interpretation of the Degradation of the Fill Factor of Silicon Heterojunction Solar Cells Due to Incomplete Hole Collection at the a-Si:H/c-Si Thermionic Emission Barrier

Abstract

An electro-physical interpretation for the degradation of the Fill Factor in p+/n silicon heterojunction solar cells (SHJ) due to incomplete hole collection at the thermionic emission barrier at the amorphous/crystalline silicon (a-Si:H/c-Si) hetero-interface is proposed supported by results of AFORS-HET device simulations. Under illumination, reflected holes at the thermionic barrier pile up at the hetero-interface which strengthens the dipole with the negative dopant ions in the doped a-Si:H(p+) layer and enhances the electric field passing through the a-Si:H layer. Such an enhanced electric field sweeps back the free holes spilling over in the intrinsic a-Si:H(i) layer from the a-Si:H(p+) layer considerably depleting the double a-Si:H layer and enhancing its resistance and the overall cell series resistance. Therefore, the degradation due to incomplete hole collection at the thermionic emission barrier under illumination can be assimilated to the effect of a series resistance does not affect the cell open circuit voltage but degrades only its fill factor. The resistance enhancement is found to be bias-dependent and to increase with decreasing the doping level in a-Si:H(p+). Predictions of the proposed model for different hole reflection probability at the barrier and for different thicknesses of the intrinsic a-Si:H(i) layer agree perfectly with the results of simulations.

The Role of Silicon Heterojunction and TCO Barriers on the Operation of Silicon Heterojunction Solar Cells: Comparison between Theory and Experiment

“…The size of the thermionic emission barrier depends on the energy band discontinuities and positions of the Fermi levels on both sides of the heterointerface, which in turn are highly controlled by doping levels. It has been reported earlier that the inclusion of a thermionic emission model as the main transport mechanism across the heterointerface will lead to an enhanced V SC which is a result of hole pile-up and reflection at the barrier [13,24].…”

“…From previous investigations [7], it has been established that any factor negatively impacting the inversion layer would have a detrimental impact on cell performance causing mainly the cell's open-circuit voltage V OC to drop. In addition, the electric field that is established around the a-Si:H/c-Si interface extends into the a-Si:H(i) buffer layer and can reach into the a-Si:H(p + ) layer depending on the amount of active doping inside that layer [13]. At thermal equilibrium, the electric field established causes a majority carrier spillover into the intrinsic buffer layer by balancing the diffusion of holes towards the emitter.…”

“…International Journal of Photoenergy barrier at the a-Si:H(i)/c-Si heterointerface and the Schottky barrier established at the a-Si:H(p + )/TCO interface [13,15] . The simulation study here is conducted over two parts.…”

“…Under thermal equilibrium, spillover of free carriers from the heavily doped a-Si:H(p) layer into the a-Si:H(i) layer under the presence of an equilibrium electric field created at the heterointerface would establish a relatively high free hole concentration within the a-Si:H(i) layer maintaining a low series resistance in the dark. On the other hand, under illumination, some photogenerated holes cannot cross the thermionic emission barrier and hence reflect and pile up at the heterointerface [13]. The pile-up of holes close to the heterointerface due to the thermionic emission barrier will cause an additional electric field that would deplete the a-Si:H(i) buffer layer and hence increase the layer resistance.…”

“…It has recently been demonstrated that the FF is influenced by properties of the thermionic emission barrier that is established at the a-Si:H/c-Si interface due to energy band discontinuities. This barrier plays a major role in the performance of SHJ devices as demonstrated by means of theoretical device simulations which revealed that properties of the thermionic emission barrier such as the Richardson constant, barrier height, and temperature can impact the FF since this barrier becomes critically responsible for carrier transport across such interfaces [12,13]. Other models have also suggested an additional transport mechanism related to quantum tunneling [14,15].…”